The disclosed invention is in the field of liquid chromatography, especially in the field of high temperature liquid chromatography (HT-LC). Liquid chromatography is used to analyze, among other materials, polymers, with regard to molecular size, by Size Exclusion Chromatography (SEC), and, with regard to chemical composition, by High Performance Liquid Chromatography (HPLC). This disclosure relates to HT-LC of polymers, and, in particular, olefin-based polymers, with regard to chemical composition.
Olefin-based polymers (such as polymers and copolymers comprising polymerized ethylene monomer and/or propylene monomer) have long been analyzed with regard to chemical composition distribution by Temperature Rising Elution Fractionation (TREF), Crystallization Analysis Fractionation (CRYSTAF), and Crystallization Elution Fractionation (CEF). However, TREF, CRYSTAF, or CEF cannot be used to analyze amorphous polyolefin polymers. Furthermore, both TREF and CRYSTAF require a relatively long analysis time. Therefore, skilled persons turned to HPLC, in an attempt to reduce analysis time, and to expand the scope of analysis to amorphous polymers. Macko et al., apparently were the first to do so in 2003, by studying the retention of polyethylene standards on silica and zeolite stationary phases (J. Chrom. A, 1002 (2003) 55). Wang, et al., studied the retention of polyethylene and polypropylene by zeolites in 2005 (Macromolecules, 38 (2005) 10341). Heinz and Pasch used a silica stationary phase to analyze polyethylene—polypropylene blends by HPLC (Polymer 46 (2005) 12040). Albrecht, et al., used a silica stationary phase to analyze ethylene-vinyl acetate copolymers by HPLC (Macromolecules 2007, 40, 5545). Albrecht, et al., used a silica stationary phase to analyze ethylene-propylene copolymers by HPLC (Macromol. Symp. 2007, 257, 46).
Some chromatography separations using graphite are disclosed in the following references: Macko et al., Separation of Propene/1-Alkene and Ethylene/1-Alkene Copolymers by High-Temperature Adsorption Liquid Chromatography, Polymer 50 (2009), 5443-5448; Macko et al., Separation of Linear Polyethylene from Isotactic, Atactic, and Syndiotactic Polypropylene by High-Temperature Adsorption Liquid Chromatography, Macromolecules (2009), 42, 6063-6067; Roy et al., Development of Comprehensive Two-Dimensional High Temperature Liquid Chromatography×Gel Permeation Chromatography for Characterization of Polyolefins, Macromolecules (2010), 43, 3710-3720; and Ginzburg et al., High-Temperature Two-dimensional Liquid Chromatography of Ethylene-Vinylacetate Copolymers, Journal of Chromatography A, 1217 (2010), 6867-6874; Miller et al., Separation of Polyolefins Based on Comonomer Content using High-Temperature Gradient Adsorption Liquid Chromatography with a Graphitic Carbon Column, Journal of Applied Polymer Science, Vol. 123, No. 2, 1238-1244; Cong et al., A New Technique for Characterizing Comonomer Distribution in Polyolefins: High Temperature Thermal Gradient Interaction Chromatography (HT-TGIC), Macromolecules, 2011, 44, 3062-3072; and Kern et al., Adsorption from Solution of Long-Chain Hydrocarbons onto Graphite: Surface Excess and Enthalpy of Displacement Isotherms, Journal of Colloid and Interface Science, Vol. 75, No. 2, 1980, 346-356. See also U.S. Publication No. 2010/0093964 (U.S. Pat. No. 8,076,147B2), and U.S. Publication No. 2011/0152499.
Additional chromatographies using graphite and/or polyethylene crystallization studies are disclosed in the following references: Chitta et al., Elution Behavior of Polyethylene and Polypropylene Standards on Carbon Sorbents, Journal of Chromatography A, 1217 (2010) 7717-7722; Findenegg et al., Adsorption from Solution of Large Alkane and Related Molecules onto Graphitized Carbon, Carbon Vol 25, No. 1, (1987), 119-128; and Yin et al., Theoretical Study of the Effects of Intermolecular Interactions in Self-Assembled Long-Chain Alkanes Adsorbed on Graphite Surfaces, Surface and Interface Analysis (2001), 32, 248-252, and Magonov, Annealing and Recrystallization of Single Crystals of Polyethylene on Graphite: An Atomic Force Microscopy Study, Journal of Macromolecular Science, Part B: Physics, 45, 2006, 169-194; and Tracz et al., Unusual Crystallization of Polyethylene at Melt/Atomically Flat Interface: Lamellar Thickening Growth Under Normal Pressure, Polymer, 47, 2006, 7251-7258.
A remaining problem for the HPLC analysis of polymers, and in particular, olefin-based polymers, is the limited separation efficiency obtained by the prior art methods (Cong et al., Macromolecular Symposia, 312, 108 (2012)). There remains a need for new chromatographic methods that provide improved separation efficiencies and reduced analysis times. These needs and others have been met by the following invention.